1. Field of the Invention
This invention relates generally to a winding structure for a dynamoelectric machine core, and more specifically, this invention relates to the structure of a superconductive winding with cooling passages formed therein.
2. Description of the Prior Art
It is known that when certain materials, referred to as superconductors, are cooled to near absolute zero they exhibit a complete loss of electrical resistance. Practical utilization of the zero resistance character of superconductive materials has been applied to great advantage in dynamoelectric machinery. For example, in a synchronous generator the use of a superconductive direct current field winding allows a considerable increase in the field magnetomotive force generated by the winding and greatly increased flux densities in the active air gap of the machine. This increase in flux density provides considerably increased power density and substantial reductions in the weight and volume of the machine. Thus, higher ratings for turbine generators can be obtained without prohibitive increases in frame size.
Superconductors which are suitable for such high current density, high field applications are subject to instabilities where a small disturbance in operating conditions can cause a quench. In particular, the superconductive effect will be quenched or lost unless the superconductors are maintained at very low temperatures. Therefore, it is imperative that adequate cooling arrangements be provided. Thus, when a winding or coil if formed of superconductive wires, provision must be made for bringing a coolant or refrigerant into intimate contact with the superconductors.
Prior art cooling arrangements for superconductive windings generally include loosely bundled windings or tightly bundled windings having flow separators of various types. The simplest of such conventional winding structures is a winding potted solidly in an insulating material. Internal heat generation is removed by conduction to a cooling source external of the winding. Strips of material with high thermal conductivity may be potted into the winding to improve the conduction heat transfer. This type of construction inherently resists conductor motion, which is a potential source of substantial losses. The helium coolant may be confined to small radii to minimize the temperature rise due to rotational compression. The major drawback of this construction is that the maximum internal heat generation rate and maximum winding thickness are inherently limited. This limits the rate at which the winding may be charged and discharged and the severity of transients which can be tolerated.
An alternate cooling arrangement involves pool boiling. By making the winding porous and immersing it in a bath of liquid coolant, the maximum internal heat generation rates can be substantially higher than those in a conduction cooled winding as described above. This allows fast recharging and discharging of the winding and increases the tolerance of the winding to other transients. However, it is more difficult to construct a porous winding with sufficient rigidity to resist conductor motion. In a rotating machine the coolant is not confined to small radii and rotational compression effects are not insignificant.
It is obvious that the design of a practical superconducting winding involves a compromise between conflicting requirements. In the cooling design of a superconducting winding the following criteria must be considered:
a. the temperature difference between the maximum conductor temperature at any part of the winding and the coolant must be minimized;
b. the internal space required by the cooling system must be minimized;
c. coolant mass present inside the winding structure at any time must be minimized for reasons of safety;
d. the coolant flow rate must be minimized for economy;
e. the cooling structure must be reliable and operate without need for internal maintenance;
f. the cooling system must be easy to manufacture and install; and
g. the cooling system and the winding must withstand the large rotational and magnetic forces involved without any mechanical movement or vibration.
As discussed above, various cooling systems have been proposed or are being used, including porous separators, conductors with internal or external cooling passages, magnets with an annulus either at the outer radius or at the inner radius of the magnet for a cooling passage, and loosely bundled windings with random cooling passages between the conductors. These existing systems generally fail to meet one or more of the above criteria completely or in part. Examples of such prior art winding arrangements may be seen in: U.S. Pat. Nos. 3,559,126 issued to Drautman, Jr.; Pat. 3,501,727 issued to Kafka; Pat. 3,444,307 issued to Kafka; Pat. 3,416,111 issued to Bogner; and U.S. Pat. 3,363,207 issued to Brechna.